[request] [en/raku] More Details on Raku Grammars

The Raku document covers regexes fairly well. But, it stops short of explaining grammars. One of Raku's major selling points is the power of its text-parsing grammars. Unfortunately, they're also not the easiest language feature to wrap your mind around — even if you already have a decent understanding of regular expressions.

If the subject is too dense, it might be worth splitting it up: have a primer on grammars inside the Raku document and then develop a second document devoted exclusively to Raku grammars. This wouldn't be without precedent, [PCRE](https://learnxinyminutes.com/docs/pcre/) has its own document; albeit a short one.